For example, a turbine wheel rotating with receiving an exhaust gas from an engine rotates at a high speed (e.g., the rotation number per minute being hundreds of thousands) under a high temperature (e.g., under a high temperature of about 950° C.) and thus, it is required to be excellent in high-temperature strength properties.
Therefore, as a material of the turbine wheel, an Ni-based superalloy excellent in high-temperature strength properties, particularly an Ni-based cast alloy including Inconel 713C and MAR-M246 as a representative has been mainly used.
As strengthening mechanisms for high-temperature strength in the Ni-based superalloy, solid solution strengthening and γ′ phase (gamma prime phase) precipitation strengthening have been used. Since the γ′ phase (a phase of Ni3(Al, Ti, Nb) that is an intermetallic compound) precipitating as a strengthening phase is stable up to a high temperature, it is difficult to produce the turbine wheel by forging and thus, usually, the turbine wheel is mainly produced by casting using an Ni-based cast alloy and is used in a state as cast.
Incidentally, in a rotating body such as the turbine wheel, as the weight of the part increases, inertial weight increases and, for example, response at the rise of rotation becomes slow. Therefore, it is requested to be light in weight, that is, low in specific gravity.
In the Ni-based alloy using the solid solution strengthening and the γ′ phase precipitation strengthening as strengthening mechanisms as mentioned above, the high-temperature strength is improved with an increase in the addition amount of solid solution strengthening elements. However, since specific gravity increases, it is difficult to cope with the request for a decrease in the specific gravity.
It is also considered to decrease the specific gravity while maintaining the high-temperature strength by decreasing the addition amount of the solid solution strengthening elements and, on the other hand, increasing the addition amount of constituent elements of the γ′-phase. However, there is a problem in that, in the case where the precipitation amount of the γ′ phase is increased, cast cracking is prone to occur during the solidification process at casting and thus productivity becomes worse.
As described above, in the alloy to be used as a material for high temperature parts such as the turbine wheel, it is required to be low in specific gravity and excellent in castability, in addition to the high-temperature strength properties. However, there has not been provided any Ni-based alloy sufficiently satisfying these requirements yet.
Incidentally, as a prior art with regard to the present invention, the following Patent Document 1 describes an invention concerning a “nickel-based alloy”, and there is disclosed a nickel-based alloy having a composition consisting of, in terms of % by weight, Co: from 14 to 19%, Cr: from 10 to 15%, C: from 0.05 to 0.2%, Mo: from 0 to 3%, and Ti: from 3.1 to 4.5%, with the balance being Ni and unavoidable impurities, and satisfying a Ti/Al ratio of 0.85 or less. However, in this Patent Document 1, there is no disclosure regarding specific means for improving the castability and the component compositions in individual Examples are all different from those of the present invention.
Patent Document 1: JP-A-2015-101753